Rabi-nutation

If J" —> J excitation is accompanied or followed by deexcitation J —> J" in a stimulated emission process (SEP), then the population efficiency of the level can be increased considerably. It is now known [248, 347] that the process might be made more effective by applying the A-configuration scheme in which the first-step (J" — J ) excitation pulse is applied after the second-step (J — J") pulse which, at first glance, seems surprising. This process is called stimulated Raman scattering by delayed pulses (STIRAP). The population transfer here takes place coherently and includes coordination of the Rabi nutation phase in both transitions. 

If the excited state is not a bound, but a continuum state, then the situation is fundamentally different, because of photoionisation. Once an electron is excited into the continuum, it escapes rapidly from the influence of the laser field (except at the highest laser field strengths), so that cycling of the population can no longer occur. This process of escape is irreversible, so that Rabi nutation can no longer occur. The time dependence of the laser and ionisation amplitudes must then be considered. 

Fig. 5. Rabi-nutation with and without a thermal field. The crosses on the T = 3 K curve have been obtained with a velocity selected beam. The velocity distribution of the atomic beam only allows to measure for interaction times in the cavity between 30 and 140 ps.

The experimental setup described above is suitable to test the Jaynes-Cummings model describing the dynamics of the interaction of a single atom with a single cavity mode. An important requirement is, however, that the atoms of the beam have a homogeneous velocity so that it is possible to observe the Rabi nutation in the cavity directly. In a... 

We should remark that, in order to observe genuine spontaneous effects in these single atom cavity experiments it is important to control the blackbody field and to reduce the number of thermal photons in the mode well below unity (k3X/h uigf < I), which requires very low temperatures. If this condition is not fulfilled, one observes the oscillations of the atomic system in the random thermal field, which also present interesting features. A discussion of these effects, along with the effect of quantum collapse and revivals of Rabi nutation in an applied coherent field can be found in re-... 

Fig. 6 Left Echo-detected longitudinal magnetization of Fea as a function of nutation pulse length. Right Absolute-value Fourier Transform, showing contributions due to ESEEM-like effects (sharp peak) and Rabi oscillations broad peak). Adapted from [150]. Used by permission of the PCCP Owner Societies...

Use of a surfactant allows solubilization of the polyoxometalate cluster K6[Vi5As6042(H20)] 8H20 (V15) in the organic solvent chloroform. Spin echo measurements revealed a phase memory time of Tm = 340 ns, which was attributed to resonances in the 5 = 3/2 excited state of the cluster [166]. No quantum coherence was detected in the pair of 5 = 1/2 ground states [151]. By measurement of the z-magnetization after a nutation pulse, and a delay to ensure decay of all coherences, Rabi oscillations were observed. From the analysis of the different possible decoherence mechanisms, it was concluded that decoherence is almost entirely caused by hyperfine coupling to the nuclear spins. 

Fig. 7.24 (a) Optical nutation in CH3p observed with CO2 laser excitation at A = 9.7 pm. The Rabi oscillations appear because the Stark pulse lower trace) is longer than in Fig. 7.23. (b) Optical free-induction decay in I2 vapor following resonant excitation with a cw dye laser at = 589.6 nm. At the time = 0 the laser is frequency-shifted with the arrangement depicted in Fig. 7.22 by Au = 54 MHz out of resonance with the I2 transition. The slowly varying envelope is caused by a superposition with the optical nutation of molecules in the velocity group Vz = o) — (oo)/k, which are now in resonance with the laser frequency oj. Note the difference in time scales of (a) and (b) [705]... 

The difference between optical nutation and free induction decay should be clear. While the optical nutation occurs at the Rabi frequency which depends on the product of laser field intensity and transition moment, the free induction decay is monitored as a heterodyne signal at the beat frequency 0) 2 which depends on the Stark shift. The importance of these coherent transient phenomena for time-resolved sub-Doppler spectroscopy is discussed in the next section. Its application to the study of collision processes is treated in Chap.12. For more detailed information the excellent reviews of BREWER [11.43,48] are recommended. 

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